Master copy entry blacklisting

Make it so that a master copy entry can't be removed and then added back in with the same URL but referring to different content. To do this, implement a URL blacklist--once a URL has been removed from the master copy, it shouldn't be allowed back in (unless explicitly instructed by the user).

Also, make it so the user can unblacklist URLs with mdtool.

Give each URL an expiration time--a URL shouldn't be blacklisted forever; just long enough to ensure that the content isn't cached in CoBlitz.